How to remove black color and use as transparent
You don't necessarily use the Mix Shader with the alpha channel of the image. You can also use the color channel. So on the darker areas the first shader (Transparent) will dominate and on the lighter parts the second shader (Diffuse) will get a higher value. You can plug a ColorRamp before the Fac shocket to finetune the silhouette. Of course you can use images with white background, just swap the transparent and diffuse shaders.